1. Field of Invention
This invention relates to apparatus known as metal detecting apparatus and apparatus for testing metal detecting apparatus.
2. Prior Art
Metal detecting apparatus is conventionally used in the food and pharmaceutical industries, and also in other fields such as the survey of subterranean objects. A particular example is for the detection of metal which may be present, usually accidentally, in substances such as foodstuffs which may be liquid or solid and which are intended to be free of metal. The foodstuffs or other substances may be conveyed continuously, e.g. either as a stream of packages or as an unpacked but substantially continuous volume of liquid or dry material in granular form which may vary from fine powder to granules of any size convenient to the conveying means. Such substances may be carried by pipes or suitably shaped conveyor belts.
Also, metal detecting apparatus is frequently of a form in which the detecting means is based on the provision of an electromagnetic field which is arranged to surround the substance or product being conveyed. Alternatively, the electromagnetic field may be generated closely adjacent to the conveying means. Sensing means within the apparatus detects any transient distortion of the electromagnetic field which will occur if a metal object should pass through the field. In this event, a warning signal may be generated or other action may be taken as necessary or desired. However, the above detecting means are described only by way of example, and the present invention is also applicable to any other metal detecting means, e.g. apparatus in which the product is stationary.
When metal detecting apparatus is installed in its intended on line working environment, it is necessary to be able to test that it is working correctly, and preferably without stopping the flow of product. For this purpose, means are often provided for simulating the presence of unwanted metal objects. This may sometimes be done by injecting an electronic signal into the field distortion sensing system and providing means to check its subsequent response. It is also known for an operator to introduce a metal bearing probe into the electromagnetic field of the apparatus and again provide means to check the response. The disadvantage of the first method is that it does not conclusively prove that the apparatus will detect metal. Likewise, the second method is unsatisfactory since it clearly requires an operator, and its results are sensitive to positioning of the probe.